Interpretive Summary: Weed control in organic onion production is often difficult and expensive, requiring numerous cultivations and extensive hand-weeding. Economical and effective weed control options for organic onion growers are needed to reduce the extent of hand-weeding. Onion safety and weed control with mustard seed meal (MSM) derived from white mustard was evaluated in greenhouse and field trials. MSM applied at rates that controlled pigweed and barnyardgrass severely injured onions and reduced onion stand by 25% or more when applied from planting to the 1-leaf stage of onions in greenhouse trials. MSM derived from white mustard cultivars with typically high glucosinolate content inhibited weeds much more than MSM derived from a cultivar bred for low glucosinolate content. In field trials, weed emergence was inhibited my MSM applications without significant injury to onions if MSM application was delayed unitil the 2-leaf stage of onions. Higher MSM rates and multiple applications inhibited weed emergence more consistantly and to a greater extent than lower rates and single applications. MSM treatment did not reduce onion yield or size in 2008 and 2009, but in 2010 onion total yield was reduced by 29% by three sequential applications of MSM at 2.2 MT ha-1. MSM has potential to be used as a weed suppressive amendment in organic production systems, but the risk of crop injury is substantial.

Technical Abstract:
Weed control in organic onion production is often difficult and expensive, requiring numerous cultivations and extensive hand-weeding. Onion safety and weed control with mustard seed meal (MSM) derived from Sinapis alba was evaluated in greenhouse and field trials. MSM applied at 110, 220, and 440 g m-2 severely injured onions and reduced onion stand by 25% or more when applied from planting to the 1-leaf stage of onions in greenhouse trials. MSM derived from mustard cultivars ‘IdaGold’ and ‘AC Pennant’ reduced plant dry weight of redroot pigweed with an effective dose that provided 90% weed control (ED90) of 14.5 and 3.2 g m-2, respectively in greenhouse trials, whereas the ED90 of MSM from a low glucosinolates cultivar ‘00RN29D10’ was 128 g m-2, suggesting that glucosinolate content and ionic thiocyanate (SCN-) production contribute to phytotoxicity of MSM. In field trials, weed emergence, onion injury, and onion yield were recorded following single or three sequential applications of MSM from 1.1 to 4.5 MT ha-1 beginning at the 2-leaf stage of onions in 2008, 2009, and 2010. By 8 wk after treatment (WAT), onion injury following MSM sequential applications was 10% or less in all three years. Combined over 2008 and 2009, 48 and 68% fewer weeds emerged 3 WAT with MSM at 2.2 and 4.5 MT ha -1, respectively. In 2010, MSM at 2.2 and 4.5 MT ha -1 reduced the number of weeds emerged 4 WAT by 91 and 76%, respectively. MSM treatment did not significantly affect onion yield or size in 2008 and 2009, but in 2010 onion total yield was reduced by 29% by three sequential applications of MSM at 2.2 MT ha-1. MSM has potential to be used as a weed suppressive amendment in organic production systems, but the risk of crop injury is substantial.